8000 km is too long of an interval. According to my Pennzoil training at their tech facility the interval should never exceed 5000km. The oil is darker, always will be, because of all the contamination from metals, combustion, unspent fuel and detergent action. Synthetic is only meant to provide extra protection in severe duty environments like every day driving, or extreme cold and hot. I once lost all coolant in a turbo engine and the synthetic cooled and lubricated at hignway speed.
